In recent years, for the sake of substitution for petroleum resources and from growing awareness of the environment, materials utilizing renewable natural fibers have been drawing attention. Among natural fibers, cellulose fibers having a fiber diameter of 10 to 50 μm, especially cellulose fibers (pulp) originated from wood have been heretofore widely used mainly as a paper product.
In addition, ultrafine cellulose fibers, which have a fiber diameter of 1 μm or smaller, are known as cellulose fibers. A sheet or composite containing ultrafine cellulose fibers has a greatly improved tensile strength because of the remarkably increased number of contact points between fibers. Further, the fiber width is smaller than wavelengths of visible light, and thus the transparency is greatly improved. For example, Patent Document 1 discloses a fiber-reinforced composite material, wherein high transparency is maintained without being affected by temperature conditions and wavelengths, and so on, and various functionalities are imparted by forming a composite of a fiber and a matrix material. Moreover, ultrafine cellulose fibers are also known to be applicable to thickeners or the like.
When ultrafine cellulose fibers are used for a thickener, for example, ultrafine cellulose fibers are dispersed in liquid and the liquid is transported to a processing plant or the like. However, such transport disadvantageously requires high cost because a large amount of a dispersion medium is contained in the liquid containing ultrafine cellulose fibers dispersed therein. For this reason, it has been required for the liquid containing ultrafine cellulose fibers dispersed thereinto to be concentrated as much as possible for reduction of the transport cost.
Powder is known as one of concentrated forms. Techniques for powdering cellulose are conventionally known also in the field of pulp materials. For example, Patent Documents 2 and 3 each disclose a technique for producing powdery cellulose from a wood pulp sheet as a raw material by using a roller mill, cutter mill, or the like. Patent Document 4 discloses a cellulose powder suitable for diluents for compression molding. According to this document, a cellulose powder with excellent compression molding properties is obtained through spray-drying a cellulose dispersion. Further, Patent Document 5 discloses a method for producing a cellulose fine particle aggregate through spray-drying an aqueous suspension containing a cellulose fine particle and a cationic resin.